It is known that polyurethane and poly(urea)urethane coatings can be applied to a variety of surfaces to provide an elastic seal. See, e.g., U.S. Pat. Nos. 4,267,299; 4,444,976; 4,463,126; 4,546,167. The advantage of these high solids, plural component solids over lower solids paints and coatings is that each coating can be applied more quickly and provide a greater coating thickness than lower solids paints and coatings. However, the known polyurethane and poly(urea)urethane coatings suffer from several disadvantages.
Polyurethane and poly(urea)urethane coatings are often applied to substrates, particularly concrete, by spraying, in an attempt to economically provide a smooth and/or chemical resistant surface. When applied to surfaces which are not substantially horizontal and upwardly facing, the sprayed material must cure quickly in order to avoid runs, sags, and uneven coating thickness on different areas of the surface. Slow-curing compositions may nevertheless be suitable for application to sloping, vertical, or overhead surfaces if fillers which increase viscosity are added. However, the viscous compositions are difficult to pump, meter and mix, and thus are not well-suited for spray application. Further, the fillers are often particulate, and particles passing through the pump apparatus can cause excessive wear and early failure of the pump's critical metering parts.
Although another way to avoid runs, sags, and uneven coating thickness on vertical, overhead, or sloping surfaces is to increase the coating's cure rate, a fast cure rate creates heat which expands air and gases which are in the coating or which are trapped under the coating. Air is often trapped under a coating which is applied to concrete, which almost always has multiple fissures and pores in which air remains after the coating is applied. Gases can become trapped because polyurethane and poly(urea)urethane coatings include isocyanate components and the reaction of such components with water produces carbon dioxide. The expansion of the air and other gases will cause bubbles, blisters, pinholes and other imperfections in the surface of the coating.
In order to avoid trapping air underneath the coating when applying it to concrete, the concrete surface can be made smooth prior to application of the coating. This can be done by applying cementatious or epoxy grouts, or through use of primers which seal the porous concrete surface. All these surface correction methods, however, increase the time needed to apply the coating--and therefore the cost--and also suffer from additional disadvantages. Without added catalysts to increase the cure rate, or without fillers, most known polyurethane and poly(urea)urethane coatings do not adhere well enough to become more than 0.01 to 0.02 inch thick on vertical or overhead surfaces, even if multiple coatings are applied. However, thicknesses of 0.03 to 0.10 inch are usually desired when the purpose is to provide a chemically resistant coating or a coating for pipes, storage tanks or concrete containers. The thicker coatings more effectively seal and prevent corrosion of the underlying substrate. Further, if the substrate is prone to cracking (viz., concrete), the thicker coatings will tend to bridge the cracks and maintain the seal.
In the absence of catalysts, the known polyurethane and poly(urea)urethane coatings cure slowly because of their chemical composition. They generally include, as essential components, a di- or poly-functional hydroxyl terminated entity (e.g., a higher molecular weight glycol), a di- or polyisocyanate, and a chain extender or curative (e.g., a polyol and/or polyamine with a relatively low molecular weight). See, e.g., U.S. Pat. No. 4,327,204 to Oyaizu et al. (disclosing a polyamine and a polyol as the hardener composition, mixed with an isocyanate composition); U.S. Pat. No. 4,526,905 to Lucast et. al. (Disclosing dialkyl diamines for use with polyols and polyisocyanates); U.S. Pat. No. 4,581,433 (Disclosing an isocyanate prepolymer and polyalkene ethers having hydroxyl functionalities of 2 to 3, and at least one diprimary aromatic diamine); U.S. Pat. Nos. 4,267,299; 4,444,976; 4,463,126; 4,546,167; See also U.S. Pat. No. 3,428,610 to Kiebert (Disclosing an intermediate containing free NCO groups and an aromatic diamine). The known coatings are generally applied by one of two methods: "the one-shot method" or "the prepolymer method". But irrespective of which application method is used, the reaction of the essential components must be catalyzed (or fillers must be added) to achieve a sufficiently fast reaction to be suitable for application to a surface which is not substantially horizontal and upwardly facing. The resulting fast, catalyzed reaction creates excessive heat, expansion of trapped air and other gases, and the associated problems noted above.
In both the one-shot and prepolymer methods of applying coatings, two streams of material are mixed. In the one-shot method the two streams both include relatively low viscosity materials, one stream containing the di- or polyisocyante and the other a mixture of polyols and/or curing agents. In the prepolymer method, one stream includes an isocyanate terminated prepolymer and the other includes the curing agent. When using the one-shot method, additional disadvantages may result from using catalysts because the catalysts will enhance the reaction of the isocyanates with water. Thus, even if the only water source is from a humid environment, the reaction will produce carbon dioxide. In addition, water induces formation of an amine which reacts preferentially to the polyol in the curing agent with the di- or polyisocyantes. This latter reaction alters the cure stoichiometry and may result in unreacted hydroxyl molecules, which may increase the permeability of the cured coating.
Another disadvantage of catalysts is that the resulting coating is not well-suited to application of successive additional coatings, as may often be desired. The successive coatings often tend to delaminate, because the fast rate of cure depleats the reactive sites of each successive layer upon itself. Finally, a high catalyst concentrations in the final coating can accelerate its destruction by hydrolysis if it comes into contact with water or ambient moisture.
Thus, what is needed is a high solids, plural component, sag resistant, low exothermic curing coating, which does not require added fillers. The coating surface should be substantially pinhole and imperfection free, and the coating should be sufficiently thick to bridge cracks in the substrate. The coating should also cure properly and evenly in the presence of water and be chemical and corrosion resistant.